The present invention relates to a method and apparatus for processing a sequence of video or other data frames at a data encoder during a synchronization ("sync") loss. When a sync loss is detected, black or pre-stored frames are provided in sync with the pre-sync loss frames until re-synchronization has been established. Null data is provided if necessary when the post-sync loss frames are not synchronized with the pre-sync loss frames. The invention is particularly suitable for use with a digital video encoder that encodes High-Definition Television (HDTV).
The communication of digital video, audio, and other data has become increasingly common due to the improved fidelity, bandwidth efficiency, and versatility of digital signals relative to analog signals. For example, many cable television (CATV) networks deliver digital television signals to users' homes via coaxial cable, or hybrid fiber and coaxial cable networks. Moreover, in addition to television signals, various other signals may be provided to the user, including audio only, video only, including still frame video, as well as data services, including Internet data, stock or weather data, computer games, and so forth.
The television or other data is transmitted from a headend of the cable network to a decoder at the user's home, or directly to the user's home, e.g., via satellite or terrestrial broadcast, and processed by the decoder to provide an output signal having a format that is compatible with the user's television, computer or other equipment.
The television or other data may be obtained by the headend via various methods. For example, the headend may have a local library of digital television programs or other data which is stored on magnetic storage media such as magnetic tape, or optical storage media, such as compact disc, digital video disc, or laser disc. The headend may also receive digital television or other data from a transmission source, including, for example, a satellite distribution network, a terrestrial broadcast network, or a microwave distribution network.
Each frame in a video or other data signal includes timing, or clock, information that allow a video encoder to be synchronized with the signal to allow proper compression and encoding. It is problematic that a sync loss may be experienced at the video encoder due to a number of factors. For example, a change in data source, e.g., from a live television broadcast to a pre-recorded movie, may result in a sync loss, thereby causing undesirable visible or other artifacts in the transmitted data stream. Additionally, a data dropout due to a defect in a data storage medium or noise in a transmission channel may result in a sync loss.
Furthermore, a change in data source may be accompanied by a change in video clock rate. For example, the HDTV format of 1920 horizontal pixels.times.1080 active video lines.times.30 fields per second interlaced scan (e.g., 540 active video lines per field) has a clock at 74.25 MHz, while the format of 1920 horizontal pixels.times.1080 active video lines.times.29.97 fields per second interlaced scan (e.g., 540 active video lines per field) has a clock at 74.175 MHz. These HDTV formats are discussed in the document SMPTE 274M, entitled "Proposed SMPTE Standard for Television-1920.times.1080 Scanning and Analog and Parallel Digital Interfaces for Multiple-Picture Rates." It is difficult for the encoder to re-synchronize with the new video clock.
Additionally, a data buffer which receives the input data signal may overflow during a sync loss since no encoding and transmission of the received data occurs.
Accordingly, it would be desirable to provide a system for processing a sequence of video or other data frames which are input to a data encoder/compressor during a sync loss. The system should provide substitute data, such as black or pre-stored data, to a data compressor that are synchronized with the pre-sync loss data frames until synchronization is re-established. For example, for frames of video data, black frames may be provided to the compressor.
The system should accommodate a change in clock rate of the post-sync loss frames relative to the pre-sync loss frames.
The system should further accommodate post-sync loss frames which are either synchronous or asynchronous with the pre-sync loss frames.
The system should provide a null signal to the compressor, if required, during a period following the establishment of resynchronization and prior to communication of a new data frame to the compressor.
The system should manage a buffer fullness level in the data encoder to prevent overflow during a sync loss.
The present invention provides a system having the above and other advantages.